The present invention relates to a method and system for selectively transmitting power change commands used for uplink power control in wireless telecommunication systems.
Traditionally, radio communication systems have employed either Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to allocate access to available radio spectrum. Both methods attempt to ensure that no two potentially interfering signals occupy the same frequency at the same time. For example, FDMA assigns different signals to different frequencies. TDMA assigns different signals to different timeslots on the same frequencies. TDMA methods reduce adjacent channel interference through the use of synchronization circuitry which gates the reception of information to prescribed time intervals.
In contrast, Code Division Multiple Access (CDMA) systems allow interfering signals to share the same frequency at the same time. More specifically, CDMA systems xe2x80x9cspreadxe2x80x9d signals across a common communication channel by multiplying each signal with a unique spreading code sequence. The signals are then scrambled and transmitted on the common channel in overlapping fashion as a composite signal. Each mobile receiver correlates the composite signal with a respective unique despreading code sequence, and thereby extracts the signal addressed to it.
Transmit power control methods can be important to communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in CDMA and is likewise useful for TDMA. Power control techniques are important for the uplink, i.e., for transmissions from a remote terminal to the network, e.g., a base station, as well as for the downlink, i.e., for transmissions from the base stations to a remote station. Uplinks and downlinks are also sometimes referred to as reverse links and forward links, respectively.
The need for transmit power control in the uplink is recognized, for example, in current CDMA cellular systems. Uplink power control according to the TIA/EIA/IS-95-A standard is provided by a closed-loop method, in which a base station measures the strength of a signal received from a remote station and then transmits one power control bit to the remote station every 1.25 milliseconds. Based on the power control bit, the remote station increases or decreases its transmit (uplink) power by a predetermined amount.
Similar techniques are applied in TDMA cellular systems. For example, systems which operate in compliance with ANSI-136 provide for a number of different power levels at which remote stations (e.g., mobile stations) can transmit as illustrated in FIG. 1. Therein, ten different power levels are defined for each of four different classes of mobile stations. The different classes of mobile stations refer to different maximum burst power levels, e.g., Class IV mobile stations have a maximum burst power level of 0.6 Watts (on average). More importantly, according to ANSI-136, the mobile stations shall be able to set their respective output transmit power level to the level specified in a control message using the mobile attenuation codes shown in FIG. 1.
In ANSI-136 systems, quality measurements are made periodically, e.g., every second, to determine an appropriate uplink transmit power for a particular mobile station. If the determined uplink transmit power is different from the uplink transmit power currently employed by that mobile station, then a power change command is transmitted to that mobile station. More specifically, the uplink power change command is transmitted to the mobile station using the downlink fast associated control channel (FACCH). Unlike the slow associated control channel (SACCH), the FACCH is implemented in a xe2x80x9cblank-and-burstxe2x80x9d manner, which means that when control information is to be transmitted to the mobile station on the FACCH, this control information replaces the payload data (e.g., voice information) which would have otherwise been transmitted.
This difference between transmitting control information using the SACCH and transmitting control information on the FACCH will be better understood by reference to FIGS. 2(a) and 2(b). FIG. 2(a) depicts a downlink timeslot format for a burst of information which has not been xe2x80x9cblanked outxe2x80x9d by a FACCH transmission. Note that each downlink timeslot includes a 12 bit field for the SACCH, which can be used to convey less critical overhead information, e.g., short text messages. The payload data, e.g., vocie information, is conveyed in the two DATA fields. By way of contrast, FIG. 2(b) depicts a downlink timeslot format for a burst of information wherein FACCH information, e.g., a power change command, has replaced the payload data fields. Since the payload data fields comprise 260 bits in each burst, the FACCH provides a mechanism for more quickly sending control information to a mobile station as compared with the SACCH.
However, using the FACCH to transmit power change information has a price, specifically that the payload data which would otherwise have been transmitted is discarded in favor of the power change information. This, in turn, creates problems at the receive side in the mobile station, e.g., by reducing the quality of the reproduced voice signal as perceived by the mobiles station""s user.
Accordingly, it would be desirable to design systems and methods which provide power control mechanisms in such radiocommunication systems that minimize the loss of payload data due to the transmission of power change commands, while continuing to provide the benefits of power control, e.g., controlling the amount of interference generated by uplink transmissions.
These and other drawbacks, limitations and problems associated with conventional power control messaging methods and systems are overcome by exemplary embodiments of the present invention. More specifically, the Applicant has recognized that (1) approximately 80% of the system carrier-to-interference (C/I) gain associated with uplink power control has already been achieved with respect to a particular mobile station when that mobile station output transmit power is about 8 dB less than its maximum output transmit power (e.g., from power level 1 to power level 4 in the example of FIG. 1) and (2) that about 20% of the power change commands transmitted by the system for uplink power control involve power levels below this threshold.
Thus, exemplary embodiments of the present invention reduce the frequency of transmission of uplink power change commands by the system for power level commands which instruct a remote station to transmit at one of the lower power levels. This, in turn, increases the speech quality associated with the payload data received by that remote station, since the payload data is xe2x80x9cblankedxe2x80x9d out less frequently by the power change commands.
For example, according to one exemplary embodiment of the present invention, a method for transmitting a power change command to a remote station in a radiocommunication system includes the steps of: determining, in the radiocommunication system, one of a plurality of power levels at which the remote station shall transmit, selecting a time at which to transmit the power change command to the remote station, wherein the selected time is based upon the one of the plurality of power levels, and transmitting, by the radiocommunication system, the power change command at the selected time.
Even more generally, systems and methods according to the present invention can determine the frequency at which uplink power change commands are transmitted by the system as a function of the power level being conveyed in the power change commands. For example, a method for transmitting a power change command to a remote station in a radiocommunication system can include the steps of: determining, in the radiocommunication system, one of a plurality of power levels at which the remote station shall transmit, and transmitting, by the radiocommunication system, the power change command to the remote station with a frequency that is based upon the determined one of the plurality of power levels.